Dynamic response and fracture characteristics of thermally-treated granite under dynamic loading

In the process of deep underground resource development and utilisation, the response of the surrounding rock mass is affected by high temperatures and the dynamic loadings that can trigger strain bursts. Therefore, strain burst failure is highly related to the rock mass's dynamic fracture toughness and energy absorption capacities. Understanding the dynamic fracturing behaviour of rock mass is crucial for designing stable underground structures and controlling the hazard associated with strain bursts. In this paper, dynamic Mode I fracture toughness tests using a split Hopkinson pressure bar (SHPB) were conducted on Cracked Chevron Notched Semicircular Bend (CCNSCB) granite specimens to investigate the relationship between the strain burst mechanism and its dynamic fracture propagation. The tests were performed on thermally treated granite specimens (from 25 to 250 °C) under a wide range of impact velocities. The dynamic fracturing characteristics and crack propagation speeds were measured by a high-speed camera (HSC). The dynamic fracturing process and the effects of thermal damage on the dynamic fracture modes were identified by detailed image analysis. Energy partition characteristics in dynamic fracture of granite samples were quantified. The results revealed that dynamic fracture initiation toughness and energy partitions highly depend on loading rate and temperature. As the loading rate increased, the failure modes changed from axial splitting to pulverisation. Under the same dynamic loading, an increase in the temperature can exacerbate the fragmentation degree of granite. Under the same dynamic loading, the dissipated and released energies increased with increasing temperature. When the loading rate was high, the loading rate strengthening effect became remarkable, and the dynamic fracture toughness of granite increased under all temperatures. Finally, the dynamic energy mechanism of strain burst with the increased loading rate and heat-treatment temperature was further discussed.